The present invention relates to waveform equalizers for reducing transmission path distortion of digital signals used for digital broadcasting.
Digital broadcasting has been mainly used in satellite broadcasting, and recently, a wave of digitalization has been sweeping terrestrial broadcasting also. Waveform-equalizing technology for reducing transmission path distortion is essential for the terrestrial digital broadcasting. A waveform equalizer receives an input signal on which distortion caused by multipath interference has become superimposed under the influence of reflected waves and the like, and produces an output signal in which the distortion is reduced.
Hereinafter, a known waveform equalizer for terrestrial digital broadcasting will be described with reference to FIGS. 2 and 8. The equalizer in this example is used in a digital television (DTV) system employing 8-level vestigial side band (8-VSB) modulation, which has been adopted in the U.S.
FIG. 8 is a circuit diagram showing a configuration of a filter that is part of the known waveform equalizer. FIG. 2 is a diagram schematically showing a configuration of a waveform equalizer according to the embodiments of the present invention. Apart from the configuration inside the filter, the components of the inventive waveform equalizer are the same as those of the known equalizer. Therefore, FIG. 2 is also used for describing the known waveform equalizer.
As shown in FIG. 2, the known waveform equalizer includes: a filter 201 for receiving a signal input to the waveform equalizer; an adder 203 for adding up signals from the filter 201 and a filter 202 and producing an output signal; the filter 202 for receiving the signal from the adder 203 and further outputting a signal to the adder 203; and an error detector 204 for detecting an error in response to the output signal from the adder 203 and outputting the error to the filters 201 and 202.
The filter 201 or 202 in the waveform equalizer includes n taps, i.e., tap 1 through tap n, as shown in FIG. 8. The tap 1 includes: a data storing flip-flop (data storing FF) 811 for receiving an input signal in synchronization with a rising edge of a data-FF clock and storing the signal; a tap coefficient computing unit 814 for calculating a tap coefficient in synchronization with a rising edge of a tap-coefficient-computing-unit clock; a tap coefficient storing flip-flop (tap coefficient storing FF) 812 for receiving a tap coefficient in synchronization with a rising edge of a tap-coefficient-FF clock and storing the tap coefficient; and a multiplier 813 for receiving an output signal from the data storing FF 811 and an output signal from the tap coefficient storing FF 812 in synchronization with a rising edge of a multiplier clock and for multiplying these two output signals together. The taps 2 through n have the same configuration as that of the tap 1 so that the signals subjected to multiplications by the multipliers in respective the taps 1 through n are added up by the adder 808, thereby producing an output signal.
In each tap coefficient computing unit, the tap coefficient for each tap is updated adaptively based on the least-mean-square (LMS) algorithm. Each of the tap coefficients is updated based on the following Equation 1Ci(m+1)=Ci(m)−α×e(m)×di  Equation 1where i represents tap numbers 1 through n, Ci(m) is the tap coefficient Ci at the point when the mth update has been completed, Ci(m+1) is the tap coefficient Ci at the point when the (m+1)th update has been completed, α is a fixed constant called the step size, e(m) is the error contained in the output signal from the waveform equalizer at the point when the mth update of the tap coefficient has been completed, and di is data held in the data storing FF at the point when the mth update of the tap coefficient has been completed.
Specifically, in the tap coefficient computing unit 814, for example, the tap coefficient C1(m+1) is determined by subtracting a number that is the product of the step size α, which is a fixed constant obtained beforehand, the error e(m) output from the error detector 204 (shown in FIG. 2) and the data di output from the data storing FF 811, from the tap coefficient C1(m) output from the tap coefficient storing FF 812. Inputs of these values into each tap coefficient computing unit is not shown in the drawings.
In this manner, by sequentially updating a tap coefficient in each of the taps based on the LMS algorithm, it is possible to obtain an output signal in which multipath interference caused in an input signal is reduced.
If the waveform equalizer as described above is used, it is possible to obtain a DTV signal in which multipath interference is reduced, but the following drawbacks occur.
In general, a tap coefficient is frequently updated in each tap for the purpose of increasing the convergence speed and tracking speed of waveforms. If the tap coefficient is frequently updated, the tap coefficient storing FF, the tap coefficient computing unit and the multiplier also operate frequently according to the interval of the update, thereby consuming a lot of electric power.